The invention is based on a traction drive for driving, and for performing regenerative braking of a rail vehicle or a combination of rail vehicles, wherein at least two axles of the rail vehicle or of the combination of rail vehicles are respectively assigned a permanently excited synchronous motor and a traction current converter, and the traction current converter has at least one machine-side pulse current converter, and the permanently excited synchronous motor is connected at its terminals to a changeover switch in such a way that the permanently excited synchronous motor can be connected to a load circuit containing at least one load element in order to drive the pulse current converter or to perform regenerative braking.
The objective when equipping rail vehicles is to use ever more effective and lightweight drive machines. Presently, inverter-fed asynchronous machines are used as the standard drive machine. However, these machines offer little potential for further development in terms of reducing their mass and torque density and, when used in rail vehicles, these machines usually require a transmission. For this reason, increasing efforts are currently being made to develop and use permanently excited synchronous machines as vehicle drives. By virtue of its high torque density, this type of machine permits direct drives to be implemented while eliminating the transmission means; this allows the mass of the drive train to be reduced to a very high degree.
Permanently excited synchronous machines have a number of particular features compared to asynchronous technology owing to their permanent excitation. Therefore, in the case of a rotating machine, for example, it is possible to achieve a braking effect in addition to the inverter-regulated generator mode using purely passive components. The braking effect which is achieved by a permanently excited synchronous machine by connecting braking resistors is disclosed in DE 101 60 612, which defines the generic type.
When the rotating, permanently excited synchronous machine with braking resistors is connected into the circuit, a characteristic torque curve or characteristic force curve, also referred to below as a natural braking characteristic curve, is obtained as a function of the rotational speed of the synchronous motor and, therefore, also of the speed of the vehicle. The braking characteristic curve has a maximum value in its profile when plotted against the rotational speed/speed.
The problem when using the regenerative braking effect of a permanently excited synchronous motor is, therefore, that the natural braking characteristic curve does not have a constant braking torque profile when plotted against the rotational speed. This constitutes a disadvantage in the braking behavior compared to the technology with a regulated braking force which is conventionally customary.